In some patients, it looks like a chicken wing. Others have a windsock, or a broccoli.

The left atrial appendage is an odd-looking pocket of heart tissue that grows in wrinkly shapes in the human body. It’s a prime breeding ground for blood clots leading to deadly strokes, and medical device companies see a $500 million market for gadgets to permanently seal it off in patients with heart conditions.

But the quest to find a safe way to do so has followed an unusual route that produced two devices, one of which was designed in Minnesota and navigated a decade of safety testing before approval last spring. The other device, which went through a Food and Drug Administration clearance process called 510(k) requiring far less clinical testing, is now associated with half a dozen patient deaths, according to a recent FDA alert.

Just how doctors respond could position Boston Scientific Corp., which sells the Minnesota-designed device, to carve out a big piece of this promising market. It also focuses more scrutiny on ­regulatory practices.

The FDA is already facing tough questions from members of Congress about whether its 510(k) process missed signs that a different medical device called a power morcellator can spread cancer cells in women and cause deaths. The FDA is in the process of improving its approval processes, but a recent agency report says its long-standing system “reduces unnecessary burden without compromising assurances that devices marketed to American patients are safe and effective.”

On July 13, the FDA published a letter revealing previously undisclosed risks from the 510(k)-cleared left atrial appendage suture system called the Lariat, made by ­California’s SentreHeart.

Doctors say thousands of patients who have a heart condition called atrial fibrillation but don’t take blood-thinning drugs can benefit from such devices. Their quivering hearts may cause blood to pool in the oddly shaped atrial appendage and form dangerous blood clots. Such clots can travel to the brain and cut off blood supply, causing disabling or deadly strokes.

Despite that need, the July 13 advisory has sharply cut down on the use of the Lariat.

“I’m getting the sense that no one is doing them anymore,” said Dr. Greg Helmer, an interventional cardiologist with University of Minnesota Physicians who has done Lariat procedures. “I would be nervous about proceeding when the FDA comes out with a statement like that.”

That could be good news for Boston Scientific, which has heavily invested in testing and marketing Lariat’s new competitor, the Watchman.

Unlike the Lariat, which lassoes a suture around the left atrial appendage, Boston Scientific’s Watchman acts as a kind of plug for the appendage that becomes permanent over time as scar tissue forms over it inside the heart.

The Watchman was designed by Plymouth-based Atritech and sold to Boston Scientific in 2011. It has been legal to sell commercially in Europe since 2005, but it wasn’t approved for U.S. patients until last March.

Boston Scientific CEO Mike Mahoney said last month that the Watchman rollout is moving ahead of schedule, with doctors at more than 100 hospitals on pace to get training on the device by the end of this year. Executives there have predicted the market for left-atrial appendage closure devices will hit half a billion dollars internationally by 2019.

St. Jude Medical in Little Canada already sells a similar Minnesota-designed device in Europe called the Amplatzer Cardiac Plug, though it delayed plans to get FDA approval after seeing Watchman’s struggles. Last month St. Jude revealed it has begun discussions with the FDA to start clinical trials for Amplatzer technology in the United States.

Meanwhile the Lariat has been on sale in the U.S. since 2006, even though it seems to pose the same level of risk as the Watchman when used to close the left atrial appendage, according to a study published in June in the widely read medical journal JAMA Internal Medicine.

“This is one of the most complex procedures that you could perform in our field,” said Dr. Jay Giri, interventional cardiologist with the University of Pennsylvania Health System and corresponding author of the JAMA study. “The Lariat was considered an intermediate-risk device. The question now becomes, why would that possibly be the case?”

Russell Seiber, founder and CEO of SentreHeart, said the Lariat was cleared for suture placement and knot-tying in medical procedures.

“This general indication is a ‘tool-based’ closure claim and enables clinicians to close structures up to 45 mm in diameter,” he wrote in an e-mail. “SentreHeart does not market or promote the product for any claims beyond its existing indication” as a remote suture delivery device for closure.

The JAMA authors could find no other use of the Lariat in the medical literature besides left aftrial appendage closure. Seiber criticized the study for omitting data in a way that he said skewed the results negatively, even though a more recent study supports “a strong safety profile.”

Proponents of the Lariat like Dr. William O’Neill, interventional cardiologist with Henry Ford Health System in Detroit, say the adverse events associated with the Lariat stem from a steep learning curve for new users of the Lariat.

“Early on, people didn’t know how to do it very well,” O’Neill said. “They’re under a lot of pressure because there is a concern about the risk, there isn’t really a lot of good prospective scientific data, most of it is anecdotal. And there is a huge problem with the learning curve.”

He said the Lariat will remain a good treatment option for a group of patients who can’t get a Watchman, which typically requires patients to take blood-thinning drugs for several weeks after surgery.

Dr. Zoltan Turi, an interventional cardiologist in Camden, N.J., praised the Lariat’s “ingenious” approach to appendage closure in a 2013 editorial in the Journal of the American College of Cardiology. But he lamented the way that SentreHeart used the FDA’s 510(k) clearance process for lower-risk devices.

“It is an exceptionally clever technology that has followed a curious regulatory pathway, approved via a 510(k) process widely seen as a back door by the interventional community,” he wrote in the JACC editorial. “Its widespread introduction with only a rudimentary evidence base turns upside down our concept of the regulatory and clinical environment.”

SentreHeart has since announced FDA approval for a randomized trial called Amaze to begin in September that will test the safety and effectiveness of using the Lariat to close the left atrial appendage, in combination with another therapy called ablation, to treat atrial fibrillation.

That would be different from Watchman’s approval, which is to decrease the risk of stroke in patients with atrial fibrillation who can’t or don’t want to start a long-term regimen of blood-thinning drugs like Coumadin.

If executives at Boston Scientific are upset about the differing approval standards for left atrial appendage devices, they don’t say so publicly.

“Irrespective of whether there might be shortcuts, we’re not interested in taking a short cut for short-run gain,” said Dr. Ken Stein, chief medical officer in the heart-device division that includes the Watchman. “Particularly in today’s world, we have an obligation to prove that our devices are safe and effective and that they make sense for patients.”

Of course, Watchman is not without risks, either. Doctors must make sure the device will have a snug fit, since left atrial appendages vary in shape and size, from chicken wing to broccoli.

The FDA’s approved indication also says patients should be able to tolerate blood-thinners for at least a short time, which may be necessary until scar tissue permanently seals the Watchman in place.

As part of its approval, the FDA ordered Boston Scientific to monitor hundreds of Watchman patients from its previous studies for five years, and follow 1,000 of its new patients in a study for at least two years. Researchers will be watching for rates of strokes and use of blood-thinners.

“It will be interesting to see how it’s surveilled, and how rigorously it’s watched,” Pennsylvania’s Giri said.